Currently, 3rd generation cellular communication systems are being rolled out to further enhance the communication services provided to mobile users. The most widely adopted 3rd generation communication systems are based on Code Division Multiple Access (CDMA) and Frequency Division Duplex (FDD) or Time Division Duplex (TDD) technology. In CDMA systems, user separation is obtained by allocating different spreading and/or scrambling codes to different users on the same carrier frequency and in the same time intervals. In time division multiple access (TDMA) systems user separation is achieved by assigning different time slots to different users. In addition to TDMA, TDD provides for the same carrier frequency to be used for both uplink and downlink transmissions. An example of a communication system using this principle is the Universal Mobile Telecommunication System (UMTS). Further description of CDMA, and specifically of the Wideband CDMA (WCDMA) mode of UMTS, can be found in ‘WCDMA for UMTS’, Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876.
In order to provide enhanced communication services, the 3rd generation cellular communication systems are designed to support a variety of different services, including packet based data communication. Likewise, existing 2nd generation cellular communication systems, such as the Global System for Mobile communications (GSM) have been enhanced to support an increasing number of different services. One such enhancement is the General Packet Radio System (GPRS), which is a system developed for enabling packet data based communication in a GSM communication system. Packet data communication is particularly suited for data services which have a dynamically varying communication requirement such as, for example, Internet access services.
For cellular mobile communication systems in which the traffic and services have a non-constant data rate, it is efficient to dynamically share radio resources amongst users in accordance with their needs at a particular instant. This is in contrast to services with constant data rates, where radio resources that are appropriate for the service data rate can be assigned on a long-term basis, such as for the duration of the call.
In the current UMTS TDD standard, uplink shared radio resources may be dynamically assigned (scheduled) by a scheduler in a Radio Network Controller (RNC). However, in order to operate efficiently, the scheduler needs to have knowledge of the volume of uplink data that is waiting for uplink transmission at the individual mobile users. This allows the scheduler to assign resources to users who need them most. In particular, it prevents that resource being wasted by being assigned to mobile stations that do not have any data to send.
Recently, significant effort has been invested in improving specifically uplink performance for 3GPP systems. One way to do this is to move the scheduling entity out of the RNC and into the wireless base stations, communicating to wireless subscriber communication units, such that transmission and re-transmission latencies may be reduced. As a result, a much faster and more efficient scheduling can be achieved. This, in turn, increases perceived throughput by the end-user. In such an implementation, a scheduler located in the base station (rather than in the RNC) assumes control over the granting of uplink resources. Fast scheduling response to a user's traffic needs and channel conditions is desirable in improving the efficiency of the scheduling and the transmission delays for the individual wireless subscriber communication units.
Specifically, in order to achieve an efficient communication of data bits across the air interface, re-transmission of data packets that are not correctly received has been specified for most 3GPP packet data services. In such systems, data re-transmissions are commonplace. So-called hybrid schemes may also be used where signals corresponding to re-transmissions are accumulated with signals from previous transmissions of the same data in the receiver, prior to decoding, in order to iteratively improve the probability of correct decoding of the data. Hybrid and fast re-transmission schemes are typically used because the optimum link efficiency (in terms of the energy required per error-free transmitted bit following re-transmission) is achieved when the probability of error for first-time transmissions is relatively high (e.g. 10% to 50%). However, the air interface transmission delay associated with a re-transmission is very high, as it includes the delay of the acknowledgement feedback process (e.g. the delay of waiting for a possible acknowledgement before deciding to re-transmit) and of the scheduling of a re-transmission data packet.
With respect to the uplink multiple access, both FDD and TDD physical layers use spreading (using one or more of a set of so-called channelization codes) followed by a chip scrambling operation. For FDD uplink, each user is allocated a user-specific sequence for the scrambling operation, which, in conjunction with the channelization code spreading, enables the separation of the individual user signals at the base station receiver. Conversely, for TDD all users within a given cell use the same scrambling code. Users in TDD using the same timeslot are thus separable primarily by means of having different physical channelization codes.
The consequence of this difference in uplink scrambling code assignment between FDD and TDD modes is that the finite set of channelization code resources must be shared out between contending users belonging to the same TDD cell, whereas in FDD, users in the same cell can use the same channelization codes subject to some restrictions on the number of codes used and their spreading factors.
In the context of the enhanced uplink systems in 3GPP, scheduling of the user's uplink transmissions is performed by the base station. A low-latency retransmission scheme is supported in which the base station sends a fast acknowledgement indicator back to the wireless subscriber communication unit pertaining to a specific block of transmitted bits. If the transmission of the data block was received in error, the indicator is set to ‘NACK’ (Negative Acknowledgment) by the base station and upon receipt of the transmitted indicator the wireless subscriber communication unit knows that the data is to be re-transmitted. If the transmission of the data block was received without error, the indicator is set to ‘ACK’ (Acknowledgment) by the base station and upon its receipt, the wireless subscriber communication unit knows that the data sent has been correctly received and can select new data for transmission in any forthcoming scheduling grants made by the base station.
The channel used to carry the ACK/NACK from the base station to the wireless subscriber communication unit is termed the E-HICH (Enhanced Uplink Hybrid ARQ Indicator Channel). This channel is necessarily a low-data-rate channel since it carries only one bit of information for each user active in the time instant. For FDD enhanced uplink, if the wireless subscriber communication unit was not active for a particular time instant, there is no need to send an acknowledgement and no acknowledgement is sent (nor is the wireless subscriber communication unit expecting to receive one).
For FDD, the way in which the acknowledgement indicator is encoded onto the E-HICH channel is by means of assigning a user-specific sequence of length ‘40’ to each user using enhanced uplink services in the cell. Notably, the sequence is assigned for the duration of the enhanced uplink “call”. During the quiet periods between bursts of uplink transmission, the code remains assigned to a particular user and cannot be re-used by other users. This effectively limits the possible active-user population size to 40 per E-HICH. Each E-HICH for FDD uses a spreading-factor 128 channelization code and thus consumes 1/128th of the available downlink code resources (note: unlike the uplink, the scrambling code is cell-specific in the downlink direction for FDD). If the population or user-base exceeds 40, a further E-HICH must be configured, thereby consuming a further 1/128th of the available downlink code resources, and so on.
A further problem that has compounded the efficient use of valuable resources is that in recent times there has arisen a desire for “always-on” internet connectivity in which users can be held in an active state (ready to transmit or receive communication from the internet without a need to reconfigure the communication state and incur the associated transmission latency penalties). For a wireless mobile communication system, when in this “ready” state, it is thus imperative that users consume as few system resources as possible when no actual data traffic is being sent or received. This enables the number of users that may be held at any time in the ready state to be maximized.
For the FDD enhanced uplink system, when a user is in this “ready” state, each user unfortunately consumes valuable downlink code resources, since a user-specific sequence has been assigned and reserved for the transmission of the acknowledgement indicator, should the need arise.
Thus, current signaling techniques are suboptimal. For example, when only a few users are actively transmitting uplink data at any one time, and the remainder of the users are inactive, any long-term allocation of downlink code resource to each user (irrespective of their activity state) for the purposes of acknowledgement signaling is wasteful of system resources.
Hence, improved signaling in a cellular communication system would be advantageous. In particular a system allowing for the provision of an improved acknowledgement process would be advantageous.